Presently, as for a method practically used for removing nitrogen oxides in exhaust gases, a selective catalytic reduction method (SCR method) is general, in which SCR method the nitrogen oxides in exhaust gases are catalytically reduced on a catalyst by using a reducing agent such as ammonia and urea and then decomposed to nitrogen and water. Recently, as an environmental pollution caused by nitrogen oxides as represented by acid rain is getting serious worldwide, a catalyst of high performance is strongly demanded.
The exhaust gases generated at incinerators treating industrial wastes and municipal refuse contain toxic organic halogen compounds such as dioxins, PCB and chlorophenol, among which dioxins are extremely harmful even at a trace amount. Effective technology for removing them includes an oxidative decomposition method using a catalyst.
In such situations, it is urgent to develop a catalyst of good performance. The exhaust gases discharged from, for example, incinerators, power plants and refuse incinerators burning heavy oil or coal contain SO2 that is oxidized on a catalyst to SO3, which causes problems such as corroding pipes in the subsequent stage of catalyst and reacting with water and ammonia in the exhaust gases to form (NH3)HSO4 resulting in performance degradation of the catalyst. Therefore, both of high performance in removing nitrogen oxides and organic halogen compounds and a low SO2 oxidation rate are required as characteristics of the catalyst.
A catalyst of a honeycomb structure for treating exhaust gases that is composed of titanium dioxide and/or a titanium composite oxide was disclosed in, for example, JP-A-2004-943 as an effective catalyst for removing nitrogen oxides and organic halogen compounds, but did not turned out to sufficiently meet the above requirements.